1. Field of the Invention
The present invention relates to an optical communication network. More precisely, the present invention relates to a technology for adapting Optical Code Division Multiplexing to an optical communication network.
2. Description of Related Art
A network called an FTTx, for example, is known as an optical communication network. An FTTx is a communication network for connecting an accommodation device and subscriber buildings (homes, offices, and so forth) by means of optical fiber. As FTTx, FTTH (Fiber to the Home), FTTN (Fiber to the Neighborhood), FTTB (Fiber to the Building), and so forth, are known, for example. FTTx is used, for example, as an access system for connecting a subscriber computer terminal to the Internet.
As a technology for implementing an FTTx, ATM-PON (Asynchronous Transfer Mode based Passive Optical Network), for example, is known. ATM-PON is able to implement broadband communications, for which optical fiber is used as the access transmission path, at low cost.
ATM-PON connects a single accommodation device (Optical Line Terminal; OLT) and a plurality of communication terminal (Optical Network Unit; ONU) via an optical coupler. In other words, a plurality of ONU is connected to a single optical coupler. Further, the optical coupler and the OLT are connected via a single transmission path. Therefore, the transmission path linking the optical coupler and OLT is shared by a plurality of ONU. In this transmission path, an uplink direction (that is, the direction from the optical coupler to the OLT) communication channel, and a downlink direction (that is, the direction from the OLT to the optical coupler) communication channel are separated by means of Wavelength Division Multiplexing (WDM). In addition, the uplink direction communication channel and downlink direction communication channel are shared by a plurality of ONU by using Time Division Multiplexing (TDM).
Downlink direction Time Division Multiplexing is implemented by performing a simple control. The OLT sequentially transmits ATM cells in which header information for discriminating an addressed ONU is stored. Each ONU judges the destination by using the header information of a received ATM cell and then discards the ATM cells addressed to other ONU destinations.
On the other hand, uplink direction Time Division Multiplexing necessitates more complex control. This is because each ONU must control the timing for transmitting the ATM cells. When a plurality of ONU outputs ATM cells with the same timing, these ATM cells collide within the transmission path linking the optical coupler and the OLT. The information stored in these ATM cells is destroyed by this collision. Therefore, in uplink direction communication, timing control so that the ATM cells that are output by each ONU do not collide with one another.
For the reasons detailed above, ATM-PON requires equipment in order to perform uplink direction timing control. This equipment is responsible for an increase in the assembly costs and maintenance costs of the optical communication network.